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Tides

Tide animation

What are Tides

- Tides continuously raise and lower the sea

surface - Tides are long period waves
- High tides are associated with the wave crest
- Low tides are associated with the wave trough
- The vertical motion can be as much as 10-15

metres, twice a day in some areas - Currents are also associated with the tidal

processes - Incoming currents associated with rising tides

are called flood currents - Outgoing currents associated with decreasing

tides are called ebb currents

Importance of Tides

- Tides produce strong currents of up to 5 m/s in

coastal waters - Tidal currents generate internal waves over

seamounts and other topography - Tidal currents play a role in suspending bottom

sediments - The elastic Earths crust bends under the

influence of the tidal potential (up to 10 m)

Importance of Tides

- Oceanic tides lag behind the tide-generating

potential and thus produce forces that transfer

angular momentum between the Earth and the Moon - Slowly changes Earths rotation and length of day
- Causes the Moon to slowly move away from the

Earth - Keeps the same face of the Moon always facing the

Earth - Tides influence the orbits of satellites

What Causes Tides

- Gravity is the main force that generates tides
- Newtons Theory of Gravitational Attraction

States that gravitational attraction between two

bodies is - directly proportional to their masses
- Inversely proportional to the square of the

distance between the two bodies

What Causes Tides

- Tide forces are based on the gravitational

attractive force - But vary inversely as the cube of the distance

between the objects - Because of the large distances, the gravitational

attraction of large bodies, such as the sun, is

small for objects on the Earth, compared to the

Earths gravitational attraction

Lunar vs Solar Tides

- Distances
- Earth to Sun 150,000,000 km
- Earth to Moon 385,000 km
- Sun is 390 times farther away than the Moon
- Masses
- Sun 2 x 1027 metric tons
- Moon 7.3 x 1019 metric tons
- Sun is 27 million times more massive than the Moon

Lunar vs Solar Tides

- Tide Generating Forces
- Now, if the sun if 27 million times more massive

than the Moon and is 390 times farther away from

the Earth, the ratio of its tide generating force

to that of the moon is

Balance of Forces

- Inertia (in the form of the Centrifugal Force)

also plays a major role in tide generation - Acts as the force to counterbalance gravity
- Can be considered as the tendency of moving

objects to continue moving in a straight line - Gravity and Inertia lead to the creation of tidal

bulges

Balance of Forces

Gravity, Inertia and Tidal Bulges

- Gravitation attraction strongest on side facing

the moon - Causes water on near side of earth to be pulled

towards the moon - Inertial forces try to keep the water in place,

but the gravitational force is larger and a bulge

appears facing the moon

Gravity, Inertia and Tidal Bulges

- On the opposite side of the earth, gravitational

attraction is weaker - Hence the inertial (Centrifugal) forces exceed

the gravitational force - Thus the water moves away from the moon, forming

another bulge

Gravity, Inertia and Tidal Bulges

- Thus we get 2 bulges, under and opposite to the

moon - As water is a fluid, the bulges stay aligned with

the moon as it rotates - Interaction with the sun modifies the tidal bulges

Declination Effects

- The Earths tidal bulges track the position of

the moon - As the moon revolves around the Earth, its angle

increases and decreases in relation to the

equator - This is known as its declination
- The two tidal bulges track the changes in lunar

declination, also increasing or decreasing their

angles to the equator - Declination animation

Frequency of Tides

- Many locations experience two high tides and two

low tides each solar day - Solar day Time for a specific point on the earth

to rotate from an exact point under the sun back

to that same exact point (24 hours) - Lunar Day Time it takes for a specific site on

the Earth to rotate from an exact point under the

moon to the same point under the moon - 24 hours and 50 minutes long
- Extra length is because moon revolves around the

earth in the same direction as the earth rotates

around its axis, needing the extra 50 minutes to

catch up - Lunar day animation
- Earth rotates through 2 bulges each lunar day,

thus giving two high tides

Spring and Neap Tides

- Although smaller than the moon, the sun does

exert a significant tidal force - Normally expressed as a variation of the lunar

pattern - When the sun and moon are aligned, the solar

bulge has an additive effect on the lunar bulge,

creating extra high and very low tides - These are called spring tides
- When the sun and moon are at right angles to each

other, their bulges partially cancel out,

producing more moderate tides - These are called neap tides
- Spring and Neap tides occur alternatively around

every 7 days - Monthly tidal animation

Additional Orbital Effects

- There are some additional orbital effects
- The Earth has an elliptical orbit, being closest

to the sun in N.H. winter. Tidal ranges are thus

largest in January (smallest distance) - The moons orbit is also elliptical, with an 8

variance in distance, producing changes in tidal

amplitude (including strong spring tides and

weaker neap tides)

Tides

- As the bulges are offset because of the

variations in the moons declination - An island can pass through the bulge on one side

of the Earth but miss it on the other - Thus not all locations get two high tides and two

low tides of the same amplitude each lunar day

Tides

- Diurnal Tides Have one high tide and one low

tide each tidal day.

Tides

- Semi-diurnal Tides Have 2 high and 2 low tides

each tidal day, with the highs and lows having

the same height

Tides

- Mixed Tides Have 2 high tides and 2 low tides

each day - But the heights of the 2 high tides and/or the 2

low tides are different

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What Else Affects Tides

- The shape of the shoreline can strongly affect

tides - Wide continental margins can lead to increased

tide amplitudes - Mid-oceanic islands generally experience small

tides - Funnel-shaped bays in particular can dramatically

alter tidal magnitude - Local wind and weather patterns can also affect

the tides

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